Waiting Line Management

1. Waiting Line Management Chapter #12

2. Waiting Line See Page # 291 (Book-1) • One or more ‘customers’ waiting for a service. • ‘Customer’ can be: • Peoplee.g. A person waiting in line to deposit cash in a bank. • Objects e.g. A machine waiting for maintenance, Inventory waiting to be delivered. Truck waiting to be loaded etc.

3. Why waiting lines form? • Temporary imbalance between demand and capacity. • Larger arrival rate than servicing rate • Randomness/Variability • Customers usually arrive at random intervals • Variability in order lengths – some orders take longer than others

4. Effects of Waiting Line • Waiting lines are non-value added occurrences. • Waiting in lines does not add enjoyment for customers. • Waiting in lines does not generate revenue for company. • Costly to provide additional waiting space. • Possible loss of business. • Customers refusing to wait • Customers leaving • Loss of customer goodwill. • Reduction in customer satisfaction. • Congestion may disrupt other business operations.

5. Objectives of Waiting Line Analysis • To improve system Utilization • To minimize the sum of two costs • Customer waiting costs • Service capacity costs

6. Structure of Waiting-Line Problems • An input, or customer population, that generates potential customers. • A waiting line of customers. (Customers Behavior) • The service facility, consisting of a person (or crew), a machine (or group of machines), or both necessary to perform the service for the customer. • A priority rule, which selects the next customer to be served by the service facility.

7. Customer population Service system Served customers Waiting line Service facilities Priority rule Figure C.1 – Basic Elements of Waiting-Line Models

8. Finite Infinite Customer Population or Input Population Source Example: Number of machines needing repair when a company only has three machines. Example: The number of people who could wait in a line for gasoline. • Customers from an infinite source do not affect the probability of another arrival • Customers from a finite source reduce the chance of new arrivals

9. Customer Behavior • Customers are patient or impatient • Patient customers wait until served • Impatient customer behave in different ways: • Balking:When customer decides not to enter in line. • Jockeying: When customer switches to another line. • Reneging:When customer quits waiting and leaves the line.

10. The Service System • Service rate depends on the structure of service system and facility. • Structure of a service system depends on various factors such as: • Service time for customer • No. of lines • No. of service channels • No. of service phases

11. Constant Variable Service time for customer Service Times Example: Items coming down an automated assembly line. Example: People spending time shopping.

12. Service facilities (a) Single line Service facilities (b) Multiple lines No. of lines in system • A single-line keeps servers uniformly busy and levels waiting times among customers • A multiple-line arrangement is favored when servers provide a limited set of services

13. Service system Arrangement • Single-channel, single-phase • Single-channel, multiple-phase • Multiple-channel, single-phase • Multiple-channel, multiple-phase • Mixed arrangement

14. Service facility Service facility 1 Service facility 2 (a) Single channel, single phase (b) Single channel, multiple phase Examples of Service Facility Arrangements

15. Service facility 1 Service facility 1 Service facility 3 Service facility 2 Service facility 2 Service facility 4 (c) Multiple channel, single phase (d) Multiple channel, multiple phase

16. Routing for : 1–2–4 Routing for : 2–4–3 Routing for : 3–2–1–4 Service facility 1 Service facility 2 Service facility 3 Service facility 4 (e) Mixed arrangement

17. One-person barber shop/ATM use Car wash Bank tellers’ windows Hospital admissions System Arrangement examples Single Phase Multiphase Single Channel Multichannel

18. Priority Rule for waiting line • First-come, first-served (FCFS)—used by most service systems • Earlier Due Date (EDD) • Shortest Processing Time (SPT) • Preemptive discipline—allows a higher priority customer to interrupt the service of another customer or be served ahead of another.

19. Important Measures of Waiting line Analysis • System Utilization • Average Number of Customers Waiting • Average Customer Time in System • Waiting time + processing time • Average Customer Waiting Time • Typically, you don’t want to keep the customer waiting for an unreasonable amount of time • Customer Waiting Costs • Service Costs • Probability of Lost Sales • Would like to minimize

20. Decision Areas for Management to avoid waiting lines • Arrival rates • Line arrangement • Number of service facilities • Number of phases • Number of servers per facility • Server efficiency • Priority rule

21. Strategies for Waiting line management • Reduce perceived waiting time • Tell customers how long their wait will be • Magazines in waiting rooms • Music/television • In-flight movies • Filling out forms • Derive benefits from waiting • Place impulse items in service facility • Advertise other goods/services • Encourage customers to come during the slack periods.